Evidence has been recently presented indicating that adequate gas exchange can be maintained in the lung with high frequency ventilation (HFV) using reduced tidal volumes and intratracheal pressures. In pilot studies we found that blood gases in cats and rabbits could be maintained for periods of two to six hours using HFV. We observed that the optimum frequency for HFV was approximately equal to the resonant frequency of the animals respiratory system (i.e. at the frequency where respiratory impedance was a minimum). The major objective of the proposed work is to test this observation in a variety of animal species. This will be accomplished by mapping as a function of frequency (2-32 Hz) the minimum tidal volume required to maintain normal blood gas concentrations. The frequency where the minimum tidal volume is required will be compared to the animals resonant frequency. If optimum frequency for HFV is related to the system resonance frequency as we suspect, then this would provide a technique for determining a priori the frequency with which a patient should be ventilated. In addition we will determine whether or not there is an important regional ventilation/perfusion component to the enhanced gas exchange with HFV. These studies will be done using the radioactive gas Krypton 81m to determine the topographical distributions of inspired gas during HFV. The results of the proposed studies will (1) provide evidence as to the optimum frequency for HFV, (2) determine the importance of regional ventilation during HFV, and (3) provide evidence as to the mechanism by which HFV enhances gas exchange.